1. Field of the Invention
The present invention relates to an automatic focus controlling method, and more particularly to a method of automatically controlling the focus of an image focused by a taking lens by use of a linear solid state image pick-up device in an image read-out system.
2. Description of the Prior Art
The image read-out system using a linear solid state image pick-up device is a method in which a linear solid state image pick-up device consisting of a number of linearly arranged solid state photosensors is moved from one end to the other across an image focused by a taking lens. Linear solid state image pick-up device is also called a linear sensor and consists of a number of linearly arranged CCD or a photo diode array. In this kind of image read-out system, the image focus is liable to be out while the sensor moves across the image along an image plane. Therefore, it is necessary to control the focus for every image to be read out.
In order to automatically control the focus, it is known to use a contrast signal obtained by the differential or finite difference of an image signal outputed by the linear solid state image pick-up device. The focus is controlled by moving the taking lens to a position where the contrast signal takes the maximum value. There are three methods in this automatic focus controlling system generally known as total scanning method, peak holding method and hill-climbing servo method.
The total scanning method is an automatic focus controlling method in which the taking lens is moved from a sufficiently front focus position to a sufficiently back focus position and the maximum value of the contrast signal is memorized, and the taking lens is moved back to the position where the contrast signal was the maximum.
The peak holding method is an automatic focus controlling method in which the taking lens is moved from the front focus position to the back focus position and the taking lens is stopped at the position where the contrast signal is detected to be the maximum.
The hill-climbing method is an automatic focus controlling method in which the taking lens is moved in one direction at first and the contrast signals before and after the movement of the taking lens are compared with each other, and if the contrast signal level increases the taking lens is moved further in the same direction and if the contrast signal level decreases the taking lens is moved in the opposite direction, and then when the contrast signal level is decreased after said comparison the taking lens is moved back to the position where the level is decreased.
Among the above mentioned three methods, the hillclimbing method is most popular owing to its short time required for controlling the focus and its accuracy of focus control which is better than the peak holding method. However, the actual contrast signal has several sub-peaks other than the main peak at the focusing position. Therefore, in the hillclimbing method, sometimes one of the sub-peaks is detected before the main peak is detected and the taking lens is stopped at the sub-peak position.